Chirper-type personal radiation monitors (PRMs) are required to be worn by personnel working near high radiation areas such as around nuclear reactors, particle accelerators, criticality areas, etc. These devices are intended to warn personnel entering an ionizing radiation field by means of audible chirps generated at a rate proportional to the field intensity. In the normal chirper-type PRM, a Geiger-Muller (G-M) tube is normally used as the radiation detection element due to its wide range of sensitivity to ionizing radiation. The G-M tube requires the application of a high voltage, typically in the range of 400-1,000 volts, between the anode and cathode electrodes of the ionizing gas filled tube. Thus, chirper-type instruments for PRM applications employ various high voltage generating schemes which generate the required high voltage from a low voltage battery source. Typically, the battery is used to power an oscillator whose output pulses are applied to the primary of a high turns ratio, step-up transformer whose secondary is connected to a voltage-multiplier circuit to obtain the required regulated high voltage bias which is constantly applied to the G-M tube to maintain the selected high-voltage operating bias. The G-M tube is connected in series with a high value load resistor in the megohm range to limit the ionization current flow. The voltage pulses produced across the load resistor in response to ionization current pulses generated in the G-M tube upon detection of ionizing radiation events are capacitance coupled into a pulse counting circuit which registers the pulses and activates an audio transducer to produce an audible chirp when a selected number of pulses are counted corresponding to the number of detected ionizing events. Normally, these counting circuits employ pulse amplitude discrimination to prevent system noise generated pulses from interfering with true radiation event counting. One problem with this type of chirper, in addition to the battery drain required due to maintain the high voltage bias at a fixed regulated value, is that the circuit becomes paralyzed due to the relatively long recovery period of the G-M tube following the detection of an ionizing event. When an ionizing particle produces a current avalanche in the tube the resulting discharge pulse continues for some time. If another particle enters the counter tube before the discharge is complete, the pulse it should produce is masked by the preceeding one, and so on for subsequent pulses produced before the tube recovers. Thus, in high radiation fields, the separate pulses are not resolved, and hence cannot be counted. The result is that at high count rates of typically this instrument can paralyze and give false indication of a low count rate or no audible chirp.
Further, these systems require rather large batteries to constantly maintain the applied high voltage bias to the G-M tube which greatly increases the size and weight of the PRM.
Thus, there is a need for an improved PRM of the chirper type which is more reliable, especially in high radiation fields, smaller in size, lighter in weight, and provide long term operation with smaller batteries.